Most M4s currently being manufactured employ the Stoner System, and are not “gas-adjustable.”

However, several manufacturers are now producing after-market accessories which allow gas-adjustment, even on Stoner-System rifles, because some shooters insist on it!

Most other military rifles employ a gas piston, connected to an op-rod, and most of these are gas-adjustable, from the factory.

“Gas adjustment” is incorporated into a control knob on the rifle which will allow the user to change the percentage of gas (diverted from the interior of the barrel as the bullet goes past the gas-tap hole) which is subsequently used to drive the action, verses the percentage that is deemed “excessive” and is thus vented to the outside.

When a rifle, gas-piston or Stoner System, is “under-gassed,” the main symptom will be sluggish operation. In the extreme, under-gassed rifles will “short-cycle,” meaning that they are no longer capable of normal semi-automatic functioning.

With gas-adjustable rifles that are shot-cycling, increasing the percentage of gas that drives the system (while simultaneously decreasing the percentage that is vented off) will restore normal functioning.

Often, rifles that function normally when clean and well-lubricated, become sluggish as they get dry and dirty. Again, increasing the gas percentage that drives the system will keep the rifle going until it can be cleaned and re-lubricated.

Most semi-automatic, military rifles that are not gas-adjustable (which includes most M4s) are deliberately “over-gassed,” so even when dry and dirty, although they do become slightly sluggish, it is not so profound that they begin to short-cycle. They are designed so that there is always plenty of gas to insure normal functioning, despite unlubricated moving parts and excessive gas leakage.

So, is there a down-side to too much gas?

Yes, several!

Over-gassed rifles will generate more felt recoil than rifles that are correctly gassed. Probably a moot point with those chambered for 5.56×45, but sometimes a significant issue with those chambered for 7.62×51

Over-gassed rifles will experience more wear-and-tear/parts-breakage than correctly-gassed rifles.

Most significantly, over-gassed rifles will often have bolt movement so violent that the magazine cannot push-up
the next round fast enough to be fed into the chamber normally. The result is that the cycle of operation yields a chamber full of thin air, or with the bolt “over-riding” the next cartridge and stopping with lugs dug-into the middle of the top of the brass case.

Of course, bolt velocity can be adjusted via heaver or lighter recoil springs as well as via gas-adjustment. In fact, with non-gas-adjustable M4s, changing recoil springs/buffers is about the only way to address the foregoing symptoms.

The point of all this is that a reliable, serious rifle, one you want with you during exciting times, is neither over, nor under-gassed. Via gas-adjustment, or via correct selection of the recoil spring, it runs reliably through all kinds of circumstances, with different brands of ammunition, and despite continuous lack of maintenance.

Many “recreational” rifles fail this test, and will fail you at a very inconvenient moment!

Serious rifles only need apply. And, when your rifle is gas-adjustable, you need to know how and when to adjust it correctly, in order to keep it within a generous“Goldilocks Zone.”